This invention relates to a DC/DC converter and, in particular, to a step-down DC/DC converter.
In the manner which is well known in the art, the DC/DC converter is a power converter for converting a DC voltage (an input DC voltage) having a voltage level into another DC voltage (an output DC voltage) having anther voltage level. The DC/DC converter is also called a switching regulator, a switching power source, or a ringing choke converter. The DC/DC converter where the output DC voltage has the voltage level higher than that of the input DC voltage is called a step-up DC/DC converter. The DC/DC converter where the output DC voltage has the voltage level lower than that of the input DC voltage is called a step-down DC/DC converter. This invention relates to the step-down DC/DC converter.
The step-down DC/DC converter uses a transistor as a switch, switches the transistor, converts an input DC voltage into an AC voltage, steps the AC voltage down by using a transformer, and rectifiers it to convert an output DC voltage.
In general, the DC/DC converter of the type described comprises a transformer having a primary winding, a secondary winding, and an auxiliary winding. The auxiliary winding is called a feedback winding, a positive feedback winding, a feedback coil, or a base winding. A switching transistor is connected in series to the primary winding of the transformer. By making the switching transistor self-oscillation, energy is transmitted to the secondary winding. The DC/DC converter further comprises a control transistor for turning the switching transistor on or off. A feedback control circuit including a Zener diode is connected between the control transistor and the auxiliary winding.
Various DC/DC converters of the type are already known. By way of example, a switching power source for controlling current characteristics on a primary side is described in Japanese Published Patent Application (A) No. H11-98838 or JP 11-98838 A. The switching power source disclosed in JP 11-98838 A realizes stable performance which is independent of an input voltage and external factors such as environmental temperature. The switching power source uses, as the above-mentioned Zener diode, a diode having a positive temperature coefficient (e.g. a “positive temperature characteristic” where its Zener voltage becomes high when the temperature becomes high). In addition, the switching power source comprises a temperature detecting resistor for correcting variations of the current characteristics due to variations of the environmental temperature. That is, in JP 11-98838 A, the switching power source needs the temperature detecting resistor because of use of the diode having the positive temperature coefficient as the Zener diode.
On the other hand, a ringing choke converter is disclosed in Japanese Published Patent Application (A) No. H8-275523 or JP 8-275523 A. The ringing choke converter disclosed in JP 8-275523 A comprises, as the Zener diode, a diode having a negative temperature coefficient (e.g. a “negative temperature characteristic” where its Zener voltage becomes low when the temperature becomes high). In JP 8-275523 A, the ringing choke converter needs any temperature compensation because of use of the diode having the negative temperature coefficient as the Zener diode.
In addition, a DC/DC converter is disclosed in Japanese Examined Utility Model Application Publication (Y) No. S56-5507 or JP 56-5507 Y. The DC/DC converter disclosed in JP 56-5507 Y comprises a Zener diode for controlling an output voltage without the above-mentioned control transistor. The above-mentioned JP 56-5507 Y merely discloses the DC/DC converter which carries out an output voltage control on the primary side of the transformer but which does not carry out an output current control on the primary side of the transformer.